Biogas plants serve to produce biogas by way of fermentation of biomass in so-called fermenters. Depending on the type of the biomass to be fermented, fermentation can be dry fermentation or wet fermentation. The biogas contains mainly methane. As is known from European patent EP1301583 B1, during dry fermentation, the biomass that is used is dry and contains interfering substances, such as sand, rocks, and woody or fibrous components. The biomass includes biodegradable waste (organic waste of animal or plant origin that can be decomposed by micro-organisms and/or enzymes), dung (a mixture of manure and litter) or grass. The biomass ingredients can cause problems in wet-fermentation biogas plants. In particular, the biomass used for dry fermentation can be stacked, but not pumped. Although referred to as “dry” fermentation, the “dry” is only relative to wet fermentation. The biomass (also called the substrate) in dry fermentation can have a water content of up to 70%. In most cases, the biomass is moistened with a liquid before and/or during fermentation in order to start and/or maintain the necessary microbial processes, i.e., the anaerobic decomposition that takes place in the fermenters. Water enriched with suitable bacterial cultures can also be used as the liquid. Often part of the seepage (also called the percolate) is used as the liquid. The percolate is removed from a bottom section of the fermenter and is again sprinkled over the biomass.
However, by far the largest part of the percolate produced during fermentation is collected in tanks in order to be applied to agricultural fields for fertilization. Section 2.2.3 of the German Biodegradable Waste Ordinance (BioAbfV) of Apr. 4, 2013 defines mandatory “sanitation” procedures for handling percolate that is applied to agricultural fields. In particular, the percolate must be heated to a temperature of 70° C. for an hour in order to achieve the defined sanitary quality and environmental compatibility based on pathogen content, weed seeds and other undesirable components such as salmonella, clubroot and tomato seeds.
Biogas plants with solid fermenters and percolate circulation are known from GB 2407088 A as well as from EP 2275526 A2. The fermentation process in the percolate tank can be thermophilic so that the percolate is sanitized in the tank. However, because the percolate tank is part of the percolate circuit, the sanitized percolate is returned to the fermenter and is contaminated again. A permanent sanitation of the percolate can only be achieved when no more percolate is added from the fermenter or returned to the fermenter.
A method of producing liquid fertilizer from percolate is sought that does not result in recontamination of the sanitized percolate as occurs in GB 2407088 A and EP 2275526 A2. A method is sought in which percolate can be drained from the biogas plant during operation of the fermenter. Furthermore, it is an object of the invention to provide a biogas plant for implementing the method.